This invention relates to an improved method and apparatus for high energy radiography, with special reference to applications where increased quality (including contrast) is of primary importance and there is less emphasis on radiation dose or short exposure times. The principal field of application of the invention is in the production of "port radiographs" as used in megavoltage radiotherapy, but the invention may also be of value in industrial applications.
An important aspect of any quality assurance program in radiotherapy is the use of portal (or verification) films which are taken during patient treatment to verify that the radiation beam does intersect the anatomical region intended. However the obtaining of satisfactory port films in megavoltage radiotherapy presents considerable problems. They are inherently of poor quality, largely because the various body tissues (even bone) show only relatively small differences in their absorption of the high energy x-rays, i.e., little primary contrast is present in the radiation beam reaching the detector. In the recent review (L. E. Reinstein et al, Med. Physics., 11(4), 555, 1984) several thousand port films were reviewed and the authors stated that "the extent and variation in quality is staggering . . . the worst of these films are totally unreadable and many suffer from insufficient contrast, improper density, bluriness, fogging, excess grain etc." The most obvious deficiency is that anatomical structures are not shown at sufficient contrast for confident visual perception. Thus, an important requirement is to provide a higher level of secondary contrast (or contrast enhancement) in the detection (or the display) system. The enhancement required is much higher than in conventional low energy (e.g., diagnostic) radiography where considerable primary contrast is already present in the emergent x-ray beam.
The usual detection system for port radiography comprises an x-ray film (having thick, double emulsions) sandwiched between a pair of metal screens (typically lead). The latent image is generated in the emulsion not only by direct absorption of x-ray photons but also by secondary electrons produced by absorption of x-rays in the metal screens. For either process a single photon/electron will create at least one and possibly several developable grains. Ultimately this means that the contrast enhancement in the film is limited.
A number of workers have tried variations of the basic metal screen-film combination (see for example R. T. Droege et al, Med. Physics, 6(6), 515, 1979), but little significant improvement in image quality has been reported.
A number of alternative approaches have also been described in the literature. One is to make high contrast prints from the original x-ray film. This however requires additional time and resources, and basically is not practical for routine use. A more recent approach is to use image processing techniques (including contrast amplification and/or edge enhancement) involving electronic systems, to display either the original radiographs or the output of a photo-electronic detection system set to capture the x-ray image directly. These devices are only in the developmental stage but will undoubtedly be expensive, especially considering the need for at least some replication of equipment for routine use in departments with more than one radiotherapy machine.
Over the past twenty years there have been reports from several centers evaluating detectors comprising fluorescent screens in contact with x-ray film (either no-screen or screen-film types). Whether or not metal screens were added outside the sandwich, the arrangement and materials were otherwise identical with those used in conventional diagnostic radiography. Two groups claimed improved contrast but others reported that the gain was slight and was accompanied by over-riding disadvantages (e.g., poor resolution, see Droege et al, op. cit.). This practice has not gained routine acceptance.